Fix listening socket leak in AgentLocalProxy.get_connection - #2694

What

AgentLocalProxy.get_connection() opens a unix socket, binds and listens on it, accepts exactly one connection, and returns that accepted socket:

def get_connection(self):
    conn = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    try:
        conn.bind(self._agent._get_filename())
        conn.listen(1)
        (r, addr) = conn.accept()
        return r, addr
    except:
        raise

The listening socket (conn) itself is never closed on either the success path or the exception path - the except: raise doesn't help, it's a no-op for control flow (catches and immediately re-raises the same exception, identical behavior to no try/except at all). Every call leaks the listening socket's file descriptor.

Fix

    finally:
        conn.close()

Closes the listening socket unconditionally. The already-accepted connection socket (r) is untouched - accept() returns an independent fd, closing the listener doesn't affect it.

Testing

Added a regression test in tests/agent.py that drives this through a real unix socket end to end: a background thread connects while the main thread calls get_connection(), and the test tags whichever socket instance get_connection() calls bind() on (via a small socket.socket subclass swapped in for the duration of the call - unambiguous regardless of how the connector thread's own retrying client sockets interleave with it, since only the listening socket ever calls bind()), then asserts that socket is closed (fileno() == -1) while the accepted one isn't.

Verified this test fails against the pre-fix code (fileno() returns a live fd, not -1) and passes against the fix. Full tests/agent.py suite: 33 passed, 1 skipped, unchanged from before this PR. flake8/black --check clean on both changed files.

Note

#2644 is an open PR touching the same file's other bare-except: clauses (a separate, broader cleanup, unrelated to this specific leak) - flagging in case of overlap, though this PR only touches the one get_connection method.